1. Field of the Invention
The present invention relates to an optical information recording method and apparatus which can reduce the degree of quality deterioration of recorded information due to inclination of the information recording surface of an information recording medium.
2. Description of the Related Art
Conventionally, the capacity and the recording speed have increased in the technology for recording information on an optical information recording medium. In recent years, techniques of recording information at high speeds have become commonplace. For example, recording speeds of two to six times the basic speed or even higher speeds are used.
Write-once optical discs (CD-R) are commonly used as optical information recording media.
In recording information on an optical disc, a digital signal is generated by digitizing recording subject information. Pits are formed on the optical disc by applying pulse laser light from an optical pickup to the rotating optical disc based on the digital signal.
In forming pits on the optical disc by using a digital signal generated by digitizing recording subject information, the intensity of laser light that is applied from the optical pickup to the optical disc is increased during periods when, for example, the digital signal is at a high level. As a result, the recording layer of the optical disc is changed by the laser light energy and pits are formed. On the other hand, during periods when the level of the digital signal is low, low-intensity laser light that is necessary for tracking control is applied from the optical pickup to the optical disc.
The information recording surface of an optical disc may incline due to a warp or deformation of the disc. As shown in FIG. 2, in most cases inclination of the information recording surface is not uniform and gradually increases as the position on an optical disc 1 goes outward. The inclination angle .theta. of the information recording surface with respect to a plane 3 that is perpendicular to the rotation axis 2 of the optical disc 1 is generally called a tilt angle.
Where the information recording surface has no inclination, as shown in FIG. 3, laser light 5 that is emitted from an optical pickup 4 is applied approximately vertically to the information recording surface. Pits are formed by the laser light 5.
However, where the emitting direction of laser light 5 is constant and the information recording surface has inclination, the spot of the laser light 5 that is formed on the information recording surface is distorted in shape. Therefore, the illumination light quantity per unit area on the information recording surface decreases and the pit size decreases. As a result, jittering during information reproduction becomes worse.
To solve the above problem, as shown in FIG. 4, the emitting direction of laser light 5 is varied so as to be kept vertical to the information recording surface of an optical disc 1. This is done in such a manner that an inclination (tilt angle) of the information recording surface of the optical disc 1 is detected and the optical pickup 4 is inclined based on the detected tilt angle.
However, the mechanism of inclining the optical pickup 4 in accordance with the tilt angle .theta. is complex, which increases the cost of the apparatus.
There is a method for performing appropriate recording (appropriateness: .beta.) without using a tilt angle correction mechanism. That is, Japanese Patent Laid-Open No. 8-147701 entitled "Power Calibration Method of an Optical Disc Recording/Reproducing Apparatus" discloses a method for optimizing the recording laser light intensity even at an outer area (OPC: optimum power control). However, this method has a problem that the OPC takes long time. The above parameter .beta., which represents asymmetry, is given by .beta.=(A.sub.1 +A.sub.2)/(A.sub.1 -A.sub.2) in the case of CD-R, where A.sub.1 and A.sub.2 are a top-level value and a bottom-level value, respectively, of pulses of 11T.